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Discreet startup delay?
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spec:

--- Quote from: Hero999 on November 25, 2018, 09:20:11 pm ---Why not simply configure the 555 timer as a Schmitt trigger?


--- End quote ---

Nice version,  The integrating capacitor referenced to 0V is the way.

But the circuit uses nine components (including the mandatory 555 decoupling capacitor across the supply line).

By the way, as you no doubt know, the CMOS 555 would probably be better: no transition supply-line current glitches and very low quiescent current. Also a smaller capacitor and bigger charging resistor can be used.

I am not a big fan of using electrolytic capacitors for timing, except possibly tantalum, because they are lifed and have a relatively large leakage current.  Much better to use a film capacitor.

To be a real smart-arse, I would suggest that a low leakage schottky diode could be used to improve the capacitor discharge at turn off.

The BJT will give a comparatively large voltage drop (CEsat). A MOSFET would fix that issue.

There is also an indeterminate state as the power line ramps up: Q1 base may be at 0V, which will turn the PNP transistor on momentarily before the 555 sorts itself out. This is being picky of course, but it is the kind of thing that can cause odd effects.
And in safety-critical applications, it would not get past the first post because of the transistors dependency on the supply line to be off (the safe condition). In general for SC, the only dependency you can have is 0V (the no power condition).

As you may have guessed, I have had many problems with power on reset (POR) and delay power on (DPO) circuits.
In one instance a quite complex equipment with all sorts of fancy circuits got through the tests, except for a silly little DPO function which nobody thought twice about. :-[

Having said all that, your circuit is well thought out (and drawn) and would be quite suitable for most home electronic projects. The parts are dirt cheap too.
Zero999:
I agree a Schmitt trigger IC such as the 74HC14 or 74HC1G14 to save space and a MOSFET would be a better option. It would probably be cheaper too. The downsides of the Schmitt trigger IC, compared to the 555 are the threshold voltages aren't so tightly controlled and it's only rated to work off up to 6V.



Yes the dv/dt of the power supply is important for any of these simple circuits to work properly. It's also why I didn't include a supply decoupling capacitor, as I'm not convinced it would do much to help and could be a hindrance, as much as anything.  I wouldn't use any of these circuits in a safety critical application: I'd buy a safety relay with a timer function.
spec:
Nice  :-+
spec:
UPDATE (2018_11_28): This schematic is now obsolete: see reply #23 for revised schematic


Below is my latest attempt:

There are no transitions on the output during start-up and there are no dependencies on anything other than 0V... at least that is the intention. ::)

All components are elementary and can be simply analyzed and simulated.

Quiescent current consumption is around 20uA with a 5V input supply.

Turn on is held off until the input supply has reached at least 3.1V which is probably a good compromise for a 5V input supply, but the hold off voltage could be adjusted to suit other input supply voltages by changing D3 accordingly.
MiDi:

--- Quote from: spec on November 27, 2018, 01:16:22 pm ---

--- End quote ---

I suggest that this would not work properly, the leakage of Q1ce could lead to uncontrolled turn on of mosfet.
A <=1M resistor from Q1c to Vi+ would fix this.
For improved stability I would add some couple of nF between Q3gs.
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